Cineoles are terpene ethers. Cineoles have been isolated from natural sources such as oils of wormwood and eucalyptus, or made synthetically from bicyclic terpene hydrocarbons, dipentenes, isoprenes, limonene, terpin hydrate, or terpineols. 1,4-Cineole (formula 1) is a by-product of the pine oil reaction ##STR1## (the hydration of alpha-pinene under acid conditions). It occurs along with 1,8-cineole (formula 2) in about 6% of the crude product. The traditionally more desirable 1,8-cineole is also known as eucalyptol or 1,1,3-trimethyl-2-oxabicyclo[2.2.2] octane or 1,8-epoxy-p-menthane. The term cineole when used herein without numerical prefixes is inclusive of both the 1,4- and the 1,8-cineole isomers.
1,8-Cineole is a valuable aroma and pharmaceutical chemical, and much attention has been given to its synthesis, isolation, and purification. The technology of isolation of 1,8-cineole and one advantageous method for conducting this isolation is discussed in a recent patent of the present applicant, U.S. Pat. No. 4,521,608 (1985). Much less is known about preparation and isolation of 1,4-cineole, which at one time was viewed as the less valued isomer. The utility of 1,4-cineole as a synthetic intermediate has recently been recognized. For instance, it has recently been found to be a valuable intermediate for the preparation of herbicides.
If good yields could be attained, it would be particularly attractive from a cost and availability standpoint to make cineoles from pine oil which is available domestically in large quantities from the acid treatment of technical alpha-pinene. I have found that 1,4-cineole is less stable than 1,8-cineole under the acid conditions of the pine oil forming reaction but that, nevertheless, it is formed in equal or greater amounts than is 1,8-cineole. This finding indicated that there was the potential for achieving higher yields of 1,4-cineole if it could be removed from the reaction mixture as formed before any degradation could occur. It has been difficult to prepare cineole in good yield from pine oil by use of any known prior art methods, and it has been particularly difficult to prepare 1,4-cineole from that source. Fractional distillation to separate 1,4- from 1,8-cineole has been found quite difficult, if not commercially infeasible, unless means can be found for supplying a feed to the still having the 1,4- isomer substantially enriched relative to the 1,8- isomer.
The rearrangement of terpineols in the presence of mineral acid is a known procedure for making cineoles; see for example U.S. Pat. No. 1,994,131. It was shown to be necessary to control carefully the temperature, pressure, and acid concentration to avoid the undesired dehydration of the terpineol to an olefin. However, even with the selection of optimum rearrangement conditions, the yields of cineole from terpineols were quite low. In the prior art method of preparing alpha-terpineol, a solution of terpin hydrate in very dilute mineral acid or organic acid is steam distilled and the terpineol codistills with the water. However, if this essentially batchwise process is run with higher acid concentration, hydrocarbons become the major product, and little or no cineole is isolable.
The present invention is an improvement in the prior art method of preparing cineole, resulting in greater overall yields of cineole. The invention also makes possible the synthesis of the less-acid-stable isomer of cineole, namely 1,4-cineole, in much better yield than hitherto possible.